The present invention relates to systems for controlling the speed of a motor or the like in a phase-locked loop, and in particular to a control circuit for pulling the speed related frequency of the system into a predetermined range of frequencies.
Phase-locked loop controlled motor drive systems are disclosed in U.S. Pat. No. 2,809,339 using vacuum tubes as the constituent circuit elements. Such systems have however been made possible only by the use of a large number of vacuum tubes, which has rendered the system to be impractical until the LSI technology drastically reduced the system cost. The concept of phase-locked loop has consequently found extensive use in many applications including tape recorders and record players, and the like.
The phase-locked loop for controlling the speed of a motor or the like typically comprises an oscillator, a frequency divider coupled to the oscillator to provide an output signal whose frequency is an integral submultiple of the oscillator frequency, a phase comparator for making a comparison in phase between the output of the frequency divider and a signal from a tachogenerator which is mechanically linked to the motor, the latter being driven by a current responsive to the output of the phase comparator through a lowpass filter. The output from the tachogenerator is a signal at a frequency related to the speed of the motor, so that when the generator frequency is tuned to the reference phase signal from the frequency divider the motor is phase-locked to the reference frequency. The system further includes a frequency pull-in control circuit which receives the signal from the generator to detect the deviation of the generator frequency from the reference frequency and derive signals for accelerating or decelerating the motor when the latter is out of the phase-locked state. When the deviation of the motor speed is such that the system is out of the phase-locked state, the pull-in control circuit seizes control of the motor speed in place of the phase comparator in response to the deviation exceeding a predetermined value. The pull-in control circuit is particularly advantageous for improving the transient response characteristic of the system when the motor speed deviates excessively from the reference or overshoots the desired speed value due to its inertia.
The prior art pull-in control circuit comprises monostable multivibrators responsive to the signal from the tachogenerator to generate pulses at the same frequency as the generator frequency. If the generator frequency is low, it is necessary to smooth out the waveform of the output of the output of the monostable by the lowpass filter so that the time constant value of the latter must be of a substantial value. This results in a phase-locked system having a slow response time.
An approach to this problem would involve the use of a sample-and-hold circuit. However, the latter circuit requires a high impedance circuit which makes it difficult to lend itself to circuit integration. In other words, the oscillator, lowpass filter and amplifier of the system constitute an analog circuit element, while the frequency divider and phase comparator function as a digital element, and these different types of circuit elements can be located on a same IC chip only if the latter is constructed of a bipolar IC such as I.sup.2 L. However, the use of a high impedance element such as field-effect transistors to form the sample-and-hold circuit constitute a barrier to the manufacture of the bipolar IC.
Furthermore, the aforesaid monostable multivibrator and sample-and-hold circuit involves the use of a capacitor to define a time constant with a resistor and such capacitors must be located outside of the IC chip because of the difficulty which the current state-of-the-art IC technology has in achieving this co-mounting, thus resulting in a substantial number of interconnecting leads and terminals between the inside and outside of the IC chip.
In addition to the aforesaid problems, the desirability of changing the motor speed between two or more set speed values as is done with record players in response to changes in oscillator frequency would require that the time constant value of the monostable or sample-and-hold circuit, when used in the pull-in control circuit, be altered in relation to the changing motor speed so that the pull-in threshold point of the system is changed accordingly to ensure smooth transition as the system switches between the phase and frequency control modes. This is only achieved at the expense of added complexity to the pull-in control circuitry.